1. Field of the Invention
This invention is in the field of coal gasifier processes and apparatus, and particularly apparatus capable of carrying out these processes by means of cyclic compression and expansion of reactant gases into and reacted gases out of the pores of the coal or other char fuel.
2. Description of the Prior Art
Gasification of char fuels, particularly coal, has been carried on for many years by use of several differing kinds of apparatus as is discussed in some detail in, for example, reference A. The most common prior art schemes gasify coal either by a devolatilization process or removing volatile portions, or by an oxidation process of oxidizing non-volatile carbon to gaseous carbon monoxide, or by a combination of these schemes.
Coal is transformed into solid coke and coke oven gas in coke ovens via a high temperature devolatilization process. The available evidence suggests that simple evaporation of volatile coal components is an important part of devolatilization but that other processes including reactions are also important as is shown by a slight net exothermic heat of reaction for devolatilization. Other char fuels have also been commercially devolatilized in a similar manner such as wood and heavy oil.
Coal, coke and wood charcoal have been transformed into producer gas in gas producers via air oxidation of solid carbon to gaseous carbon monoxide. This overall gasification reaction can be represented by the following reaction balance: EQU 2C+O.sub.2 +3.76N.sub.2 .fwdarw.2CO+3.76N.sub.2 +(QCO)
The net exothermic heat of this reaction, QCO, is of the order of 96000 Btu per lb. mol of oxygen consumed. Where the producer gas is to be utilized elsewhere at a distance, a large portion of this appreciable net heat of reaction can be lost unless the hot producer gas is cooled as by generating steam.
Essentially this same char gasification reaction has also been carried out using pure oxygen or oxygen-enriched air, on an experimental basis, in order to reduce the content of inert nitrogen in the final product gas.
Coke and wood charcoal have been transformed into water gas via steam oxidation of solid carbon to carbon monoxide and hydrogen. This overall gasification reaction can be represented by the following reaction balance. EQU C+H.sub.2 O.fwdarw.CO+H.sub.2 +(QH)
The net endothermic heat of this reaction, QH, is of the order of 55273 Btu per lb. mol of steam reacted. Since this reaction is endothermic, it is necessary to first heat up the carbon to a high temperature before applying the steam and this cycle of preheat followed by steaming is repeated.
Combinations of air oxidation with steam oxidation are also used for gasification of char fuels. Also, the several gases created, producer gas, water gas and coke oven gas, have been blended together and with other gases, after production, to create special gas fuel properties.
A primary shortcoming of producer gas has been its low volumetric heating value (circa 120 Btu per cu. ft. at STP) due to the high content of inert nitrogen. Consequently, producer gas cannot be economically pumped through pipe lines for any great distance. At some distance, the pumping power required per cu. ft. of gas will exceed the gas heating value.
Water gas possesses an intermediate volumetric heating value (circa 280 Btu per cu. ft. at STP) due to the high content of hydrogen. Hence, water gas can be economically pumped through pipelines of moderate distance.
The gases of devolatilization possess high volumetric heating values (circa 550 Btu per cu. ft. at STP) due to the moderate content of gaseous hydrocarbons, and these gases can be economically pumped considerable distances.
A gas of high volumetric heating value is commonly and herein referred to as a "rich" gas whereas a gas of low volumetric heating value is commonly and herein referred to as a "lean" gas.
The term char fuel is used herein and in the claims to include any carbon containing fuel which is either a solid or can be transformed partially into a carbonaceous solid when devolatilized. Included as char fuels within this definition are coal, coke, wood, wood charcoal, oil shale, petroleum coke, heavy petroleum fuels such as bunker C, garbage, wood bark, wood wastes, agricultural wastes, and other carbonaceous materials, together with mixtures of these char fuels. Note that a char fuel is both an input and an output of such devolatilization processes as coke ovens and charcoal ovens.
The term oxygen and oxygen gas refer to molecular oxygen as O.sub.2 and a gas containing oxygen in appreciable quantities, such as air, is referred to as a gas containing appreciable oxygen whereas a gas, such as producer gas or water gas, containing very little oxygen, is referred to as a gas essentially free of oxygen even though it may contain appreciable portions of atoms of oxygen combined with carbon and hydrogen.
Herein and in the claims those gases put into a char gasifier scheme and into contact with char fuels therein are referred to as reactant gases whereas those gases which emerge from contact with the char fuel and are removed therefrom are referred to as reacted gases. In a gas producer, for example, air is a reactant gas and the producer gas is a reacted gas.
Much coal lies in seams too thin and too deep to be economically mined and recently some efforts have been directed to gasifying such thin seam coals in place underground. Most of these underground gasification processes admit air and other reactants into the coal seam via one borehole and extract the product of reacted gases via another borehole some distance away. Hence, throughflow of gases between boreholes is required. When air is used as reactant gas, a single reacted gas emerges which is of low volumetric heating value and hence useable only in the vicinity of the coal seam. This throughflow requirement and the low heating value of the reacted gas are among the deficiencies of prior art underground char gasification schemes.
Other deficiencies of prior art char gasification systems include: a requirement for net work input to drive pumps, blowers, etc.; loss of all or a major portion of the net heat of the gasification reaction; slowness of the gasification reaction since reaction occurs largely on only the external char surface area; loss of volumetric heating value due to occurrence of the Neumann reversion reaction where steam is used. This latter, Neumann reversion reaction, can be represented by the following reaction balance: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
and occurs principally in the absence of reducing conditions where both CO and steam are present. The resulting insert and noncondensible CO.sub.2 acts to reduce the volumetric heating value of the product reacted gas.